The present invention relates to synthetic aperture radars, and more particularly, to frequency range gate closure circuitry for use in synthetic aperture radars that improves mapping resolution.
When an airborne radar is used to form a synthetic aperture (or array) radar (SAR) for high resolution ground mapping, the uncompensated motion of the radar-bearing aircraft will degrade the map resolution. In particular, the inherent motion of the aircraft flying past the map area in very high resolution spotlight (or spot) modes can severely degrade the map resolution if motion compensation is not provided. Range gate closure is a motion compensation technique that compensates for this particular type of motion degradation. Traditional range gate closure apparatus uses time delay circuitry to adjust the listening time, or range gate, of the radar receiver to correspond to the changing aircraft range from the map area. This is referred to as time range gate closure to distinguish it from frequency range gate closure performed by the present invention.
Using the time range gate closure technique, range gate time adjustment is accomplished by incrementally adjusting the start time of a local oscillator (LO) frequency sweep that is used to downconvert radar returns in the receiver. In ASARS and ASARS MTI ground mapping systems manufactured by the assignee of the present invention, the LO sweep start time is adjusted with a coarse digital delay step size of 10 ns in conjunction with a fine analog delay step size of 625 ps. The coarse digital delay has an accuracy of less than 50 ps which is determined by the phase accuracy of the system digital clock. The fine delay is generated by a 4-bit programmable tapped delay line with a nominal step size of 625 ps and a total delay range of 9.375 ns. The 4-bit programmable tapped delay line is capable of generating 16 possible time delays from 0 ns to 9.375 ns in increments of 625 ps. The delay steps are generated by tapping an inductor and capacitor network, then multiplexing the tapped delays with a 16 to 1 analog multiplexer. The manner in which this network is tapped contributes to step size inaccuracies. Also, inductor and capacitor temperature variations and tolerances also contribute to the step size inaccuracies as well as=total delay inaccuracies. The total delay has an inaccuracy of .+-.600 ps or 12.8% of the step size, and the 625 ps step size of the fine analog delay has an inaccuracy of .+-.225 ns which is 36% of the step size. Hence, the accuracy of the delay line step size determines the accuracy of the range gate closure motion compensation.
Therefore, it is an objective of the present invention to provide for improved range gate closure circuitry for use in synthetic aperture radars that improves mapping resolution and improves upon conventional time range gate closure techniques.